Microwave intrusion detectors are known for detecting the movement of objects (preferably human) within an area of coverage. In general, a microwave signal is transmitted into the zone under surveillance and the signals reflected back from every object in the zone, are returned to a receiver. Should one of the objects in that zone be moving, then the signal received after reflection off that object will be of changing phase. The rate of change of phase is proportional to the velocity of movement of the object. By mixing this received signal with a portion of the transmitted signal, there is produced a resultant frequency equal to the rate of change of phase. This frequency is amplified and processed in order to activate an alarm when movement of objects within the zone occurs.
The disadvantage of such designs is that there is no ability to determine the type of motion of the object. Merely the existence of motion is detected. Thus, there can be no discrimination against reciprocating motion such as vibration, swaying curtains, swinging signs, etc., which are moving about a static mean point. This reciprocating motion can cause high false alarm rates, and is a different form of motion from that expected from an intruder. An intruder does not move about a static mean point but the contrast, must progress substantially unidirectionally through the zone for some distance. Existing microwave intruder detectors known to Applicant are unable to distinguish between progressive and reciprocating motion.
False alarm rates for existing designs of microwave detectors, caused by various forms of interference, are minimized by control of the bandwidth of the amplifier and by careful adjustment of threshold level, integration rate and decay time within signal processing circuitry. However, even the best designs of this type do not reduce the false alarm rate attributable to reciprocating motion.
Use of a microwave transmission medium despite the above mentioned setbacks has the advantage over ultra-sonic systems of not being affected by interference from wind or air turbulence or acoustic noise.
It is an objective of this invention to provide a microwave movement detector which can discriminate between progressive or unidirectional movement and reciprocating movement.